Complex tone modulation

ABSTRACT

The generation of signals which vary in both frequency and amplitude in accordance with the amplitude of an information bearing input signal is disclosed. The invention thus relates to a complex tone generator wherein a monitored parameter is transduced into an electrical input signal which is employed to both frequency modulate a carrier signal in the audible frequency range and to amplitude modulate the frequency modulated carrier; the manner of amplitude modulation resulting in suppression of the unmodulated carrier.

United States Patent Fein July 15, 1975 [54] COMPLEX TONE MODULATION3,054,073 9/1962 Powers 332/4] X F 3,486,l 17 12/1969 Groves et al i v332/17 X [75] lnvgmor- Harry New Haven Com 3,562,428 2/1971 Starkeyetal. e e e a 179/1 ST [73 Assigneez w p lnSu-uments Inc, N Haven93,715,474 2/l973 Calfee 332/l7 X Conn 3,778,718 l2/l973 Bass 61 al332/l7 X {22] Filed: Primary ExaminerAlfred L. Brody 21 Appl No: 392,430

[57] ABSTRACT 52 U.S. Cl. 332 17; 128 2.05 P; 128 2.05 I 1 128/205 gs/21 5/179 N 149/1 2, The generation of signals which vary in both fre-325/l38. 3'25/l52: 2 332/44 quency and amplitude in accordance with theampli- [SH Int Cl h A6lb'5/02 A6'1b 5/04 tude of an information bearinginput signal is dis- [58] Field 3'32/44 17 22 23 R closed. The inventionthus relates to a complex tone 332/23 39 generator wherein a monitoredparameter is trans- }79 N 2 A 128/265 duced into an electrical inputsignal which is em- Q 2 6 S B ployed to both frequency modulate acarrier signal in Y the audible frequency range and to amplitude modu-[56] Rderences Cited late the frequency modulated carrier; the manner ofamplitude modulation resulting in suppression of the UNITED STATESPATENTS unmodulated carrier. 2.347398 4/1944 Crosby v. 332/4] X 2.9218872/1960 Aiken 332/41 X 4 Claims, 2 Drawing Figures F 1 a 5 i g 2f 7 i M 205C. g i 1 M I 1 i a: l i l l E l i i A i 1 i COMPLEX TONE MODULATIONBACKGROUND OF THE INVENTION:

1. Field of the Invention The present invention relates to theproduction of audible signals which vary in both tone and amplitude inaccordance with the amplitude of an input signal commensurate with aparameter of interest. More specifically, this invention is directed toa complex tone generator. Accordingly, the general objects of thepresent invention are to provide novel and improved methods andapparatus of such character.

2. Description of the Prior Art For many applications it is useful, andin some cases mandatory. to provide an audible measure of an electricalsignal other than the conventionally transduced signals commensuratewith speech and music. Such an audible measure is particularly useful inthe case of electrical signals, commensurate with a monitored parameter,in the range of frequencies which are normally sub audible; i.e., thosesignals which contain frequency components from dc. to lOOHz. Lowfrequency signals of this nature are, for example. provided at theoutput terminals of an electrocardiograph (ECG) or anelectroencephalograph (EEG). An audible measure of these exemplary lowfrequency signals, and other similar signals, has substantial potentialutility as an aid to one whose vision is normally occupied elsewhere orfor the blind. Thus, by way of example, a blind physician would be ableto interpret an audible electrocardiogram. Also by way of example, asurgeon whose hands and eyes are otherwise occupied can be usefullyinformed as to the state of a patients heart, respiration and otherorgans during an operation by a device such as an audibleelectrocardiogram.

One method currently in use for pulsatile signals such as those whichcomprise an electrocardiogram encompasses the pulsing of a brief tonecommensurate with the patients heart beat. The resulting beeping" sound,while providing an indication of the presence or absence of a heartbeat, is inherently deficient in that it can not provide more subtleinformation about the shape of a given waveform which can be ofconsiderable clinical importance.

SUMMARY OF THE INVENTION The present invention overcomes the abovebriefly disussed and other deficiencies and disadvantages of the priorart by providing a modulation technique which permits a significantimprovement in the information content of an audible monitoring signal.In accordance with the present invention a carrier signal is frequencymodulated by a low frequency amplitude variable input signal.Thereafter, the resulting F.M. signal is amplitude modulated by the samemodulating input signal. The form of amplitude modulation employed issuppressed carrier modulation whereby the output signal is suppressed inthe absence ofa modulating signal and increases from zero with theamplitude of the modulating signal. The output of the modulator isapplied to an audio amplifier and audible tones will be transduced by aspeaker associated with the amplifier only when a modulating inputsignal is present.

BRIEF DESCRIPTION OF THE DRAWING The present invention may be betterunderstood and 2 its numerous objects and advantages will become ap;parent to those skilled in the art by reference to the ac companyingdrawing wherein:

FIG. 1 is an electrical circuit block diagram of a preferred embodimentof the invention; and

FIGS. 2a and 21) respectively are graphical presentations of amodulating input signal and the resultant out put signal provided by theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

Before discussing the disclosed embodiment of FIG.

I, some well known aspects of the physiology of hearing will be brieflydiscussed. It is well known that the human ear does not discriminateamplitude differences very accurately. Restated, the ear is a poormeasurer of loudness. For example, on the average a steady tone mustvary in loudness or intensity by t l.5db. for an average listener todiscern the change in loudness. On the other hand the ear is anexcellent discriminator of pitch (frequency). The present inventionexploits the sense of hearing in a way that is particularly appropriatewith the foregoing physiological principals in mind.

' Since the ear is very sensitive to changes in pitch or frequency, thefirst step in accordance with the present invention is to frequencymodulate a steady tone or carrier frequency. The carrier centerfrequency is chosen to lie in the band between 400Hz and 250OHz; thisfrequency range being that which is the optimally heard band in humanaudition. The impressed modulating signal will, in the manner well knownin the art, cause the carrier frequency to vary upwardly with increasingamplitude and to vary downwardly for decreasing amplitudes.

The frequency modulated signal resulting from the above described firststep is amplitude modulated by the same modulating input signal. Inaccordance with the present invention the output of the amplitudemodulator is suppressed to nearly zero in the absence of a modulatingsignal and increases from zero with the amplitude of the modulatingsignal; i.e., suppressed carrier modulation is employed and the A.M.modulator functions as a multiplier.

As a third step in accordance with the invention, the output of thesuppressed carrier modulator is applied to an audio amplifier and thenceto a sound transducer. Since the carrier signal is suppressed, audibletones will be heard only when a modulating input signal is applied tothe system. The steps of frequency modulation and suppressed carrieramplitude modulation are performed essentially simultaneously and anaudio signal having a chirping" quality will accordingly result. Thepresent invention enhances the discernability of the informationcontained in the modulating input signal in two ways. First, theinvention exploits the ears normally good pitch acuity. Secondly, bysuppressing the carrier, the invention enhances the ears ability todiscriminate amplitude by providing a quiet background.

With reference now to FIG. I, the electrical signal to be modulated isapplied at input terminal 10. The input signal is weighted by the pairof attenuators l2 and 14. The function of this weighting is theproportioning of the input signal which is to be F.M. modulated and theportion which is to be A.M. suppressed carrier modulated. Thus, theweighting attenuators l2 and 14 determine the index of modulation. Theweighting attenuators l2 and 14 may be linear attenuators, such aspotentiometers, or may be non-linear devices, such as silicone diodechains, which non-linearly weight the input signal.

The output signal of attenuator 12 is applied as the modulating input toan F.M. oscillator 16. The F.M. oscillator 16 may be any commerciallyavailable F.M. oscillator and in one reduction to practice of theinvention oscillator 16 comprised a multivibrator, functioning as asquare wave oscillator, having its output signal applied to a shapingcircuit which provided an essentially pure sine wave. It will beunderstood that, while the output of oscillator 16 is preferably a sinewave, other waveforms could be employed.

The output of oscillator 16 is applied to a first input terminal of amultiplier, indicated generally at 18, which functions as the A.M.suppressed carrier modulator. The weighted signal from attenuator 14 isapplied as a second input to multiplier 18.

A circuit suitable for use as the multiplier 18 has been shownschematically. To understand the operation of this circuit it must berecognized that the collector current of most junction transistors is afunction of the product of the transconductance and the base inputvoltage; the transconductance in turn being a function of collectorcurrent. Thus, if one voltage signal ofa pair of such signals is causedto vary the collector current and the second signal is applied to thetransistor's base, the resulting collector current will be a function ofthe product of the two voltage signals. in the disclosed embodiment thesignal from oscillator 16 is capacitively coupled to the base oftransistor T1 while the signal from attenuator 14 is capacitivelycoupled to the base of transistor T3. Transistor T3 is connected as anemitter follower current source for transitors T1 and T2. The outputvoltage, measured as the voltage drops across load resistors R1 and R2respectively of transistors T1 and T2, with an input signal applied toterminal 10, will be the product of the voltages applied to multiplier18 from oscillator 16 and attenuator l4. Transistor T2 serves tocompensate for the effects of temperature changes on transistor T1. Theresistors in the circuit which have not been specifically identifiedare, in the manner well known in the art, employed for d.c. biasing.

To summarize, the frequency modulated signal from oscillator 16 and theweighted signal from attenuator 14 are applied to multiplier 18.Multiplier 18 generates the product of these two applied signals; theoutput of the multiplier being delivered as the input to audio amplifier20. The output of audio amplifier 20 is employed to drive a loudspeaker22. The product of the inputs to multiplier 18 will be zero if an inputsignal is absent at terminal 10, since no signal will appear at theoutput of attenuator circuit 14, and no audible sound will emerge fromthe loudspeaker even though oscillator 16 is continuously running.

Referring to FIG. 2a, the wave shape of an assumed electrocardiogramsignal is depicted. The resultant modulated wave shape, as would appearat the output of multiplier 18 if the waveform of FIG. 2a was applied toinput terminal 10, is depicted in FIG. 2b. It is partic ularly to benoted that both the frequency and ampli- LII LII

tude of the output signal of FIG. 2b increases with increasing inputsignal magnitude and decrease with decreasing input signal magnitude.

As should now be obvious to those skilled in the art, the presentinvention provides a complex output waveform which, when applied to aloudspeaker, produces an audibly distinctive signal which can be easilyinterpreted by trained listeners. The invention exploits the sense ofhearing by varying the frequency of the carrier and by suppressing thebase carrier tone so that amplitude modulation effects are enhanced. Theresulting sound wave has a unique sound with subtle variations which arecapable of a wide range of variation. Thus, with experience, a user maylearn the voice" of unique events.

While a preferred embodiment has been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. By way of example, the inputsignal applied to terminal 10 may be derived from any transducer such aspressure, position and temperature sensors. Accordingly, it is to beunderstood that the present invention has been described by way ofillustration and not limitation.

What is claimed is:

l. A method for the production of audible signals which vary in bothtone and magnitude in accordance with the amplitude of a monitoredparameter, the amplitude of the monitored parameter varying at asubaudible frequency, said method comprising the steps of:

transducing a monitored parameter which varies in amplitude at asub-audible frequency into an electrical information bearing signal;

frequency modulating a carrier signal in the audible frequency range inaccordance with the amplitude of the information bearing signal;

directly amplitude modulating the frequency modulated carrier signalwith the information bearing signal while simultaneously suppressing thecarrier signal in the absence of an information bearing signal; and

transducing the amplitude modulated signal into an audible signal.

2. The method of claim 1 wherein the step of amplitude modulatingcomprises:

multiplying the frequency modulated carrier signal by the informationbearing input signal.

3. The method of claim 1 further comprising:

proportioning the information bearing input signal whereby a portionthereof is employed for frequency modulating the carrier signal and aportion thereof is employed for amplitude modulating the frequencymodulated carrier signal.

4. The method of claim 2 further comprising:

proportioning the information bearing input signal whereby a portionthereof is employed to frequency modulate the carrier signal and aportion thereof is multiplied with the frequency modulated carriersignal.

1. A method for the production of audible signals which vary in bothtone and magnitude in accordance with the amplitude of a monitoredparameter, the amplitude of the monitored parameter varying at asub-audible frequency, said method comprising the steps of: transducinga monitored parameter which varies in amplitude at a sub-audiblefrequency into an electrical information bearing signal; frequencymodulating a carrier signal in the audible frequency range in accordancewith the amplitude of the information bearing signal; directly amplitudemodulating the frequency modulated carrier signal with the informationbearing signal while simultaneously suppressing the carrier signal inthe absence of an information bearing signal; and transducing theamplitude modulated signal into an audible signal.
 2. The method ofclaim 1 wherein the step of amplitude modulating comprises: multiplyingthe frequency modulated carrier signal by the information bearing inputsignal.
 3. The method of claim 1 further comprising: proportioning theinformation bearing input signal whereby a portion thereof is employedfor frequency modulating the carrier signal and a portion thereof isemployed for amplitude modulating the frequency modulated carriersignal.
 4. The method of claim 2 further comprising: proportioning theinformation bearing input signal whereby a portion thereof is employedto frequency modulate the carrier signal and a portion thereof ismultiplied with the frequency modulated carrier signal.